The following is not an admission that anything discussed below is prior art or part of the common knowledge of persons skilled in the art.
Biofiltration is an air pollution control technique that has been used to control odour and remove volatile organic compounds (VOC) from waste gas streams generated by wastewater treatment plants and chemical plants, as well as various rendering, food processing, flavour manufacturing and composting facilities.
In a typical biofilter, a waste gas stream is urged to flow through a moist, biologically active, packed bed. The bed contains microorganisms that are immobilized on a thin biofilm that is formed on the surface of the packing material. The microorganisms serve as the biocatalysts in the contaminant degradation process. They transform air contaminants into biomass and other products through their metabolic activities.
The process underlying the operation of a biofilter is a multi-step process that involves phase transfer, adsorption and biodegradation. As a first step, contaminants are transferred from the gaseous phase to the liquid phase. Once in the liquid phase, the contaminants are adsorbed to the biofilter media (also referred to as packing material). Thereafter, the contaminants are biodegraded within the biofilm. The overall efficiency of the biofiltration process is affected by the relative rates of phase transfer, adsorption and the biological reactions.
Several different biofilter media have been used in the past. These may fall in one of two categories: naturally bioactive, and inert. For both types of media, biofilters are typically sized to provide sufficient contact time with the biofilter media in order to achieve phase transfer, adsorption, and biodegradation of odorous compounds. This contact time is referred to as the empty bed residence time, or EBRT. Generally, EBRTs between 30 and 120 seconds are common. However, for both naturally bioactive and inert biofilter media, it has been found that EBRTs below 45 seconds may cut into required safety factors in order to maintain reliable odor treatment (Easter et al., 2006).
Bioactive packing materials may include soil, peat, compost, bark and manure. These materials can retain water and generally contain enough nutrients to sustain an initial microbial population. These materials have been used in many applications because they tend to be abundantly available and are generally inexpensive. However, this type of biofilter media has encountered various drawbacks in the field. Biofilters using these materials tend to require large filter beds on account of the low biodegradation rate and the significant bulk density of the media that tends to limit the filter bed height. Additionally, these media tend to degrade over time. They lose their water retaining characteristics and settling of the media due to biomass growth tends to occur. Eventually, biofilters using this type of media may experience a loss of performance due to a significant gas phase pressure drop in the media and channeling of the waste gas through the filter bed.
Inert biofilter media are porous materials (either naturally occurring or synthetic) that usually require inoculation of microorganisms. Examples of inert biofilter media that have been used in previous biofilter applications include wood chips, activated carbon, gas-aerated concrete, gravel, lava rock, ceramics and polymeric foams. Some synthetic biofilter media have yielded better contaminant removal rates and generally performed better than bioactive packing materials. This is due in part to the fact that they tend to have a larger surface area and have been able to achieve a better distribution of gas flow through the media. However, clogging, compaction and excessive gas-phase pressure drop due to extensive biomass growth still remain a serious problem for these types of biofilter media. These issues can severely impact performance of the biofilter causing a decline in the contaminant removal efficiency.
An example of a synthetic filter media is described in European Patent No. 0 497 214 (Fattinger). The biofilter media has a hydrophilic core coated with a hydrophobic layer. The hydrophilic core is populated by microorganisms. The core is a granular material made from a porous substance, such as gas-aerated concrete, swelling clay or pumice, whereas the hydrophobic layer can be activated charcoal or adsorption resin. A bonding agent may also be used when applying the hydrophobic layer to the hydrophilic core. Fattinger also discloses that this biofilter media may be used to purify exhaust air containing toluene, xylene, ethyl acetate and benzene. This packing material tends to suffer from clogging problems.
A biofilter system using the packing material of Fattinger to remove hydrogen sulfide from waste gas streams was described in U.S. Pat. No. 6,358,729 (Ferranti). Ferranti describes a plant for the depuration of air polluted with odorous substances, such as hydrogen sulfide, mercaptans and dimethyl disulfide. The plant includes a prescrubbing section, a filtering bed and post-scrubbing section, all placed in sequence. The filtering bed of Ferranti preferably consists of particles of a filtering material made in accordance with Fattinger. The compact plant has a high empty bed residence time (EBRT) for H2S removal at high concentrations. In addition, it was found not to be particularly efficient in removing recalcitrant reduced sulfur compounds from the waste gas streams.
United States Patent Publication No. 2005/0084949 (Shareefdeen) describes another synthetic biofilter media, which is currently made commercially available by the assignee of the present application, BIOREM Technologies Inc. of Guelph, Ontario under the name BIOSORBENS™. This media has had greater success in removing hydrogen sulfide from waste gas streams. The biofilter media has a porous hydrophilic nucleus and a hydrophobic coating on the hydrophilic nucleus. The hydrophilic nucleus is formed of aggregates whose primary ingredients preferably include silica and alumina. The hydrophobic coating includes a metallic agent, microorganisms, nutrients, organic carbon, an alkaline buffer, a bonding agent, an adsorptive agent, and a hydrophobic agent. The inclusion of a metallic agent (preferably iron) in the biofilter media allows the removal of sulfur by the formation of iron sulfide and also serves to enhance the conversion and biological processing of sulfur compounds in the contaminated air. The metallic agent acts as catalyst to increase the rate of biological oxidation and enhance the activity of the microorganisms.
The removal of reduced sulfur compounds from waste gas streams presents a major challenge. While biofilters have been used successfully to remove hydrogen sulfide from waste gas streams, certain reduced sulfur compounds, such as mercaptans and the methyl sulfides (especially dimethyl disulfide) have tended to resist such treatment on account of their relative stability and low solubility in aqueous solutions.